As to a plasma treatment apparatus, an electron lithography apparatus, an ion implantation apparatus, and the like, which are used in a manufacturing process of a semiconductor device, and an ion doping apparatus and the like, which are used for manufacturing a liquid crystal panel, it is required to securely hold a semiconductor wafer, a glass substrate, or the like, which is an object to be treated, without causing damage thereto. In particular, contamination of the semiconductor wafer, the glass substrate, or the like, to be treated should be controlled stringently these days, hence most of the systems which have been conventionally employed for clamping the substrate mechanically are being replaced with the electrostatic chuck systems that utilize an electrostatic attraction force. This is because a material of the clamp itself (aluminum material in many cases) is exposed to plasma during the treatment to be spattered therefrom, or ion irradiation in the ion implantation treatment causes the clamp to be spattered, which lead to generation of suspended matter. If the suspended matter falls onto the retained substrate, the semiconductor device or the liquid crystal display panel is significantly affected, for example, in characteristic, yield, or the like.
FIG. 5 is a schematic cross sectional view of a typical electrostatic chuck apparatus, which illustrates a conventional example of an electrostatic chuck apparatus. The electrostatic chuck apparatus includes a metal base 1 made of aluminum, copper, stainless steel, or the like, an electrostatic chuck 7 formed on an upper surface side (substrate w side) of the metal base 1. The electrostatic chuck 7 includes a lower insulating layer 4, an attracting electrode 5 and an upper insulating layer 6. As an example of use of the electrostatic chuck apparatus, when the substrate w including a semiconductor wafer or the like is subjected to etching treatment in, for example, a plasma apparatus, the temperature of the substrate w temperature increases up to approximately 200 to 400° C. Accordingly, it is necessary to cool down the temperature of the substrate under the treatment to an appropriate temperature. For this reason, a liquid conduit 2 is usually formed inside the metal base 1 so as to allow a liquid cooling medium such as pure water to flow therethrough. The liquid cooling medium supplied externally via a heat exchanger and the like (not shown) is circulated inside the metal base, to thereby cool the substrate w. In addition to indirectly cooling the metal base 1 by cooling the substrate w as described above, other means are adopted in which cooling gas, such as helium, supplied from the lower surface side of the metal base 1 is supplied to the back surface side of the substrate w through a gas supply path 3 provided to the metal base 1, to thereby cool the substrate w directly.
In order to employ the cooling means of the latter, that is, in order to send the cooling gas supplied from the lower surface side of the metal base 1 to the back surface side of the substrate w attracted to the electrostatic chuck 7 so as to cool the same, gas supply path outlets (gas exhaust holes) 3a having a diameter of approximately 1 mm are usually formed in the upper surface side of the metal base 1. After forming the electrostatic chuck 7 on the upper surface side of the metal base 1, through holes are formed from the upper insulating layer 6 side of the electrostatic chuck 7 to the gas supply path outlets 3a, so as to allow the cooling gas to be supplied toward the back surface of the substrate w. Here, it is required to supply the cooling gas uniformly to the substrate w, and hence the gas supply path outlets 3a on the upper surface side of the metal base 1 may be formed as many as approximately 200 in the case of the metal base 1 for attracting a semiconductor wafer having a diameter of 300 mm, for example.
Meanwhile, when forming the electrostatic chuck 7 on the upper surface side of the metal base 1 having a gas supply path outlet 3a formed therein, if the lower insulating layer 4 is formed by thermal spraying of a ceramic powder, for example, a part of the thermal-sprayed ceramic powder may be deposited in the gas supply path 3 via the gas supply path outlets 3a. The deposit 10 in the gas supply path 3 may prevent the cooling gas from being jetted out, making an amount of gas to be jetted nonuniform when the electrostatic chuck apparatus is used. In addition, the deposit 10 maybe jetted out together with the cooling gas toward the substrate w side and may become a source of contamination. Therefore, the inventors of the present invention have proposed a method of bonding an insulating plate to each gas supply path outlet 3a of the metal base 1 before forming the lower insulating layer 4 (see Paragraph 0010 in Patent Document 1). However, the work of bonding the plate to every gas supply path outlet 3a requires enormous efforts than expected, so it is necessary to improve the technique further.    Patent Document 1: JP 2004-349664A